User Manual

2400 Series SourceMeter
®
User’s Manual Range, Digits, Speed, and Filters 7-9
Filters
NOTE For the Pulse Mode of the Model 2430, filtering is not used, and there-
fore, cannot be enabled. Pressing the FILTER key results in the “Invalid
in Pulse Mode” message.
Filter lets you set the filter response to stabilize noisy measurements. The
SourceMeter uses a digital filter, which is based on reading conversions. The dis-
played, stored, or transmitted reading is an average of a number of reading con-
versions (from 1 to 100).
There are two averaging filter types to choose from: repeating and moving (Figure
7-1). For the repeating filter (which is the power-on default), the stack (filter count)
is filled, and the conversions are averaged to yield a reading. The stack is then
cleared, and the process starts over. Choose this filter for sweeping so readings
for other source levels are not averaged with the present source level.
The moving average filter uses a first-in, first-out stack. When the stack (filter
count) becomes full, the measurement conversions are averaged, yielding a read-
ing. For each subsequent conversion placed into the stack, the oldest conversion
is discarded. The stack is re-averaged, yielding a new reading.
When the filter is first enabled, the stack is empty. Keep in mind that a filtered
reading is not yielded until the stack is full. The first reading conversion is placed
in the stack and is then copied to the other stack locations in order to fill it. Thus,
the first filtered reading is the same as the first reading conversion. Now the nor-
mal moving average filter process can continue. Note that a true average is not
yielded until the stack is filled with new reading conversions (no copies in stack).
For example, in Figure 7-1A, it takes ten filtered readings to fill the stack with new
reading conversions. The first nine filtered readings are calculated using copied
reading conversions.
NOTE Sweeping with moving average filter enabled is not recommended as it
may yield incorrect results. The desired readings may be changing with
the sweep source values and these changes can be skewed by the aver
-
aging technique.
Response time considerations
The filter averaging mode and count affect the overall reading speed. The moving
averaging filter is much faster than the repeat averaging filter because the unit
does not have to refill the filter stack for each reading. Also, the number of read-
ings averaged will affect reading speed; as the number of readings averaged
increases, the reading speed decreases.